Method of electroless plating

ABSTRACT

A thin, discontinuous film of a catalyst is deposited on a surface to be plated. The surface is rinsed with a solution which includes a wetting agent for the surface, and is then electrolessly plated with a metal.

Umted States Patent 1 1 1111 3,877,981 Arnold 5 1 Apr. 15, 1975 4] METHOD OF ELECTROLESS PLATING 2,556,540 5/1951 11611 1 96/50 R 3,399,268 8/1968 Schn'eble, Jr. et al. ll7/2l2 -1 Invent Amhmy Franc Arm), Rmges1 3,607,379 9/1971 Leinkran 117/212 3,632,435 1/1972 12111155 11 117/212 Assignee: Corporation York, 3,674,550 7/1972 Mallory...., 7/212 [22] Filed: 1973 Primary Examiner-John D. Welsh I [21] Appl. No.: 355,720 Attorney, Agent, or Firm-Glenn H. Bruestle; William S. Hill [52] US. Cl. 427/305; 106/1 [51] Int. Cl. B44d l/l8 [57] ABSTRACT [58] Fleld of Search 1 17/212 130 A thin, discontinuous film of a catalyst is deposited on a surface to be plated. The surface is rinsed with a so- References Cimd lut1on wh1ch mcludes a wettmg agent for the surface,

and is then electrolessly plated with a metal.

6 Claims, No Drawings 1 METHOD OF E acrrto Es's PLATING BACKGROUND OF THE INVENTION c,

When a metal such as nickel is electrolessly deposited on a surface which is not normally a catalyst for such deposition, the .surfacemust first be activated by treating it with a catalyst. Although a number of metals catalyze the electroless deposition of nickel, the one mostoften used is palladium. A very thirifdiscontinuous film of palladium particles is deposited asa catalyzing medium by reducing a solution of a palladium salt which is in contact with the surface to be plated. If the palladium film is non-uniform, a non-uniform layer of nickel results.

When the palladium film is first deposited under conventional, carefully controlled-conditions, it is usually substantially uniform. But,- during the conventional step of rinsing with deionized water between deposition of the palladium and the subsequent nickel plating, non-uniformity of the palladium is often introduced, and this then causes difficulties with the nickel plating operation. There are often areas where nickel does not deposit and others where the nickel is too thin or too thick.

During the rinsing step, the palladium-coated surface usually becomes dewetted. When the coated surface is lifted out of the rinsing bath, droplets of water form on the surface and these tend to pull the palladium into clumps. Also, when the coated surface is immersed in the nickel plating bath, the palladium particles are further moved around on the surface. Too much nickel may be deposited on the areas where the palladium is "clumped" and too little or none where the palladium is missing.

The present invention is based on the discovery that, if a wetting agent of a particular type is included in the rinsing solution which is applied between the palladium deposition step and the nickel plating step, the original palladium deposit is not disturbed during the rinsing operation and uniformity of the nickel plating is considerably enhanced. The wetting agent must be one which does not interfere with the nickel plating operation.

DESCRIPTION OF PREFERRED EMBODIMENT ing techniques followed by etching away the unwanted areas of silicon dioxide, The remaining hardened photoresist has then been removed.

in preparing the slices to be nickel plated on the electrode contact areas, they are etched lightly with a solution such as the following:

Glacial acetic acid 950 ml/liter 48% HF 50 ml/liter by wt. PdClg 2 ml/liter "The amount of PdClsolution may be varied between 1 about 05 and ml/liter. If too much PdCl is present, the'deposit of palladium metal tends to become nonuniform and this leads to'non-uniformity in the nickel 5 deposit.

The amount of 48% HF can be varied between about ml and 150m]. The glacial acetic acid and the hydrofluoric acid improve the wetting of the silicon sur-' face by the -palladium solution and thus permit more uniform deposition of palladium at low concentration. This requires less palladium.

The slices are immersed in the above catalyst solution for 20 seconds at C. The time can be varied'between about 5 seconds and 60 seconds. The solution is reduced by the silicon and palladium deposits mostly on the silicon and not on the silicon dioxide. However, some palladium does occasionally deposit on the oxide.

Next, the slices are rinsed with a solution which contains an agent which wets silicon. An example of this 20 solution is:

2 propanol deionized water The slices are immersed in this solution for 25 seconds at 25C. The propanol is used as a wetting agent. Other wetting agents, such as other lower molecular weight alcohols (i.e., up to C which do not interfere with the subsequent nickel deposition, may be substituted. Higher molecular weight alcohols can also be used. The presence of the wetting agent prevents water droplets from forming and thus it prevents the rather loosely held palladium from being moved around on the surface. If one of the lower molecular weight members of this series is used, (e.g., propanol or isopropanol) the amount used should be at least about 10% by volume of the rinsing solution. As the molecular weight rises, the amount of the alcohol used can be correspondingly less.

After treatment with the rinse solution, the slices are immersed in a nickel plating bath which may have a composition such as the following, per liter of solution. The solvent used is deionized water.

Sulfuric acid to pH 4.8

The slices are immersed in this solution for 3 minutes at 80C. The glycolate and the acetate are chelating agents. Other chelating agents may be used. The ammonium acetate is included to complex any palladium salt that may have been dragged over from the palladium catalyst solution despite thorough rinsing. The glycolate complexes the nickel. Any soluble nickel salt may be used and the concentration is not critical. Any conventional reducing agent for nickel may be used in place of the hypophosphite. Other examples are amine boranes and hydrazine. The pH range may be either 3.8 5.1 or 7.5 10.5.

If the surface being plated isnot one which is capable of reducing the catalyst ion, a coating of a sensitizing agent, such as tin, must first be deposited. This must be ladium, in an aqueous solution containing a wetting agent that does not interfere with the subsequent electroless deposition of said metal on said surface.

2 A method according to claim 1 in which said metal is nickel.

3. A method according to claim 2 in which said wetting agent is a low molecular weight alcohol.

4. A method according to claim 3 in which said alcohol is propanol.

5. A method according to claim'4 in which said propanol is present in an amount of at least about 10% by volume of the rinsing solution.

6. A method according to claim 1 in which said substrate is silicon. 

1. IN A METHOD OF ELECTROLESSLY DEPOSITING A METAL CATALYZED BY A FILM OF PALLADIUM ON A SUBSTRATE SURFACE, COMPRISING: DEPOSITING A THIN, DISCONTINUOUS FILM OF PALLADIUM ON SAID SURFACE, RINSING SAID SURFACE, AND ELECTROLESSLY DEPOSITING A METAL ON SAID SURFACE, THE IMPROVEMENT COMPRISING RINSING SAID SURFACE, AFTER DEPOSITING SAID FILM OF PALLADIUM, IN AN AQUEOUS SOLUTION CONTAINING A WETTING AGENT THAT DOES NOT INTERFERE WITH THE SUBSEQUENT ELECTROLESS DEPOSITION OF SAD METAL ON SAID SURFACE.
 2. A method according to claim 1 in which said metal is nickel.
 3. A method according to claim 2 in which said wetting agent is a low molecular weight alcohol.
 4. A method according to claim 3 in which said alcohol is propanol.
 5. A method according to claim 4 in which said propanol is present in an amount of at least about 10% by volume of the rinsing solution.
 6. A method according to claim 1 in which said substrate is silicon. 